Integration of the Cell Cycle with Signal Transduction from the Mating Factor Pathway. This signal transduction pathway inhibits G I cyclin function. G I cyclins CLN1 and CLN2 (but not CLN3 or B-type cyclins, including CLB5) have the reciprocal activity of inactivating the mating factor pathway. We are investigating the mechanism of this reciprocal regulation, which is likely due to regulation of the PAK-related protein kinase Ste20. Mass Spectrometry is being used to map and monitor the phosphorylation of Ste20 and mutants of Ste20 under a variety of conditions